Steam turbines



A. FRANKEL Feb. 22, 1966 STEAM TURBINES Filed Oct. 19, 1964 UnitedStates Patent 3,236,051 STEAM TURBINES Adolf Frankel, Altrincham,England, assignor to Associated Electrical industries Limited, London,England, a British company Filed Oct. 19, 1964, Ser. No. 404,650 Claimspriority, application Great Britain, Oct. 24, 1963, 42,026/63 6 Claims.(Cl. 60-64) This invention relates to steam turbines having asubatmospheric pressure stage provided with a low pressure steam sealinggland to reduce to a minimum the leakage of air from the atmosphere tothe turbine. Such a low pressure sealing gland generally comprises atleast one annular or similar channel provided in the region of thecasing of the sub-atmospheric stage which is exposed to the atmosphereand which surrounds the turbine rotor shaft. Sealing steam is suppliedto the gland at a pressure and in a quantity suflicient to ensure thatthe amount of sealing steam discharged through a clearance, at thegland, between the rotor shaft and the casing is adequate to preventingress of air through the clearance and into the turbine.

The sealing steam should be at a temperature within the range 220 F-260F., at a pressure slightly above atmospheric pressure, and shouldpreferably be slightly superheated to avoid the danger of condensationin the gland and of condensate entering the turbine.

The sealing steam is conveniently derived from a main turbine boiler orfrom steam discharged from a steam sealing gland in a high pressurestage of the turbine; steam derived from both these sources is howeverat an excessive temperature and an excessive degree of superheat fordirect use in the low pressure gland and must accordingly bedesuperheated.

Desuperheating may be effected by means of a heat exchanger disposedwithin a turbine main condenser. Such an arrangement produces widevariations in the temperature of the sealing steam as a result ofvariations in turbine loading and/or running conditions. Alternativelydesuperheating may be effected by injecting a spray of condensate intothe superheated steam flowing to the sealing gland, or by passing thesuperheated steam into heat exchange relationship with cooling tubesconveying a flow of cooling liquid. In both cases the rate of sprayinjection or cooling liquid iiow, as the case may be, must beautomatically controlled in order to maintain the temperature and degreeof superheating of the sealing steam reasonably constant. In the eventof a fault occurring in the automatic control the temperature and degreeof superheat of the sealing steam will violently lluctuate; if thefault, in particular, results in overcooling of the sealing steam,condensation of the steam in the sealing gland may occur and the suddenentry of condensate from the gland into the turbine may have verydamaging eifects.

An object of the present invention is the provision of an improved steamturbine in which the temperature and' the degree of superheat of sealingsteam supplied to a low pressure sealing gland are suitably stabilisedregardless of turbine loading and without resort to automatic control.

According to the present invention a steam turbine having asub-atmospheric pressure stage provided with a low pressure steamsealing gland, includes a desuperheater distinct from a turbine maincondenser and arranged to bring superheated steam from a suitable sourceinto heat exchange relationship with the condensate of steam dischargedfrom the turbine before the steam enters the sealing gland, when theturbine is in operation.

lCC

The invention will now be described by way of example with reference toFIGURES l and la of the accompanying drawing which schematically shows asuitable desuperheater for supplying sealing steam to a low pressuresealing gland in a steam turbine.

Referring to FIGURE l of the drawing the steam turbine includes asub-atmospheric pressure cylinder 1 provided with a low pressure sealinggland 2 at the region of the cylinder casing 3 which is exposed to theatmosphere and which surrounds one end of the rotor shaft 4. A highpressure sealing gland 14 is provided at the other end of the casing 3.

Pipelines 5 and 6 connected respectively to the inlet and outlet of thegland 2 feed sealing steam to and collect steam discharged, from, thegland 2. Pipelines 15 and 16 respectively feed steam to and collectsteam discharged from, the high pressure gland 14. The inlet and outletof each of the sealing glands 2 and 14 comprises suitable manifolds andare not shown in the drawing.

The pipeline 5 is connected to one end of a heat exchange coil 7 sealedinto the lower end of a chamber S, the other end of the coil 7 beingconnected to a boiler supplying superheated steam to the turbine and tothe high pressure sealing gland 14. The pipeline 6 is connected to theupper end of the chamber which has, also at its upper end, a vent 9 opento atmosphere. In this case when the turbine is in operation the boilingpoint temperature of the condensate cannot exceed about 212 F.

A heat exchange coil 10 arranged to convey a ow of a suitable coolingliquid is sealed also into the upper en-d of the chamber 8. The coolingliquid flowing through the coil is in this embodiment feed water for theturbine boiler.

With the turbine in operation steam discharged from the gland 2 entersthe chamber 8, through the pipe line 6, is condensed by heat exchangewith the coil 10 and falls to the lower end of the chamber 8 where it ispermitted to reach a maximum level indicated by the dotted line 11, tocompletely immerse the heat exchange coil 7. The level of condensate isprevented from rising above the level 11 by an overflow actuatingmechanism in the form of a ball valve 12 which operates a drain valve13; the drain valve can also be used to completely empty the condensatefrom the chamber 8.

Superheated steam from the turbine boiler passes through the heatexchange coil 7, is desuperheated by heat exchange with the condensatewhich is kept boiling by the desuperheating heat transfer and passesinto the gland 2 through the pipe line 5. The steam released by -thisboiling action is again condensed by heat exchange with the coil 10.

By the use of the desuperheater arrangement described above, airabsorption into the condensate is prevented and corrosion in the steamsystem of the turbine is correspondingly reduced. Furthermore thetemperature of steam discharged from the coil 7 and entering the gland 2will only slightly exceed and cannot fall below, the boiling point ofthe condensate at the pressure existing in the chamber 8 and will varyonly little irrespective of turbine loading and running conditions, andirrespective of the steam quantity flowing through the heat exchangecoil 7.

In an alternative embodiment of the invention as shown in FIGURE la thevent 9 at the upper end of the chamber 8 includes, for example, apressure release valve or a restricted orice 17 to maintain the pressurein the chamber above atmospheric pressure. In this case the boilingpoint temperature of the condensate in the chamber 8 is maintained above212 F. and further reduces the risk of condensation in the gland 2.

In a further embodiment of the invention, the chamber 8 and the heatexchange coils 7 and 10, are replaced by three enclosures defined bythree concentric tubes or hollow spheres, the inlet and outlet of eachenclosure passing 'sealingly through any adjacent surrounding enclosure.

`pressure in the rst enclosure is always slightly above atmosphere. Suchan arrangement, however, may be more prone to instability.

What I claim is:

1. A steam turbine comprising:

(a) a superheated steam source,

(b) a sub-atmospheric pressure stage,

(c) a low pressure steam sealing gland provided in the sub-atmosphericstage and effective to reduce to a minimum ingress of air from theatmosphere and into the stage,

(d) a desuperheater distinct from a turbine main condenser andcomprising heat exchange means, arranged to condense steam discharged`from the turbine and desuperheating means arranged to bring superheatedsteam from the source into heat eX- change relationship with thecondensate before the steam enters the sealing gland.

2. A steam turbine according to claim 1, wherein the desuperheatercomprises:

(a) a first enclosure having its inlet and outlet connected respectivelyto a steam discharge point on the turbine and atmosphere,

(b) a second enclosure in heat exchange relationship with the rstenclosure and arranged to convey a flow of cooling liquid whereby toeffect the condensation of steam passing into the first enclosure, and

(c) a third enclosure arranged to be in heat exchange relationship withthe condensate collecting in the rst enclosure and having its inlet andoutlet connected respectively to the steam source and the inlet of thesealing gland.

3. A steam turbine according to claim 2, wherein the rst enclosure isconnected directly to atmosphere so that the condensate in thedesuperheater is maintained under atmospheric pressure whereby toprevent its boiling point temperature exceeding about 212 F.

4. A steam turbine according to claim 2, wherein the first enclosure isconnected indirectly to atmosphere so that the condensate is maintainedabove atmospheric pressure whereby to permit its boiling pointtemperature to exceed 212 F.

5. A steam turbine according to claim 1, wherein the condensate isderived from steam discharged from the sealing gland.

6. A steam turbine according to claim 4, wherein the rst enclosure hasits inlet connected to the outlet of the sealing gland so that thecondensate isderived from steam discharged from the sealing gland.

No references cited.

SAMUEL LEVINE, Primary Examiner.

ROBERT R. BUNEVICH, Examiner.

1. A STEAM TURBINE COMPRISING: (A) A SUPERHEATED STEAM SOURCE, (B) ASUB-ATMOSPHERIC PRESSURE STAGE, (C) A LOW PRESSURE STEAM SEALING GLANDPROVIDED IN THE SUB-ATMOSPHERIC STAGE AND EFFECTIVE TO REDUCE TO AMINIMUM INGRESS OF AIR FROM THE ATMOSPHERE AND INTO THE STAGE, (D) ADESUPERHEATER DISTINCT FROM A TURBINE MAIN CONDENSER AND COMPRISING HEATEXCHANGE MEANS, ARRANGED TO CONDENSE STEAM DISCHARGED FROM THE TURBINEAND DESUPERHEATING MEANS ARRANGED TO BRING SUPERHEATED STEAM FROM THESOURCE INTO HEAT EXCHANGE RELATIONSHIP WITH THE CONDENSATE BEFORE THESTEAM ENTERS THE SEALING GLAND.